LIMESTONE 169 



it is usually sufficient to determine the content of carbonate carbon 

 (or carbon dioxid) and compute from this the equivalent amount 

 of calcium carbonate. Of course, this computation would show 

 that 100 pounds of pure dolomite would be equivalent to about 

 109 pounds of pure limestone. 



Agricultural writers have placed upon record the general opinion 

 that magnesian lime is very likely to produce injurious effects when 

 used upon the soil. 



In his comprehensive and very valuable treatise upon "The 

 Agricultural Use of Lime," Doctor William Frear includes the fol- 

 lowing comments (Report Pennsylvania State College, 1899-1900, 

 pages 14 to 176) : 



"Lloyd states that lime (CaO) is the only material of value in burnt lime 

 and applies the adjective ' bad ' to a lime containing 60 per cent of lime (CaO) 

 and 30 per cent of magnesia (MgO). Low says of the magnesian limestone of 

 England: 'If applied after being calcined, in the same quantity as other limes, 

 it produces a temporary sterility, burning, as it were, the soil ; hence, it is termed 

 hot lime and is applied in much smaller quantity than other kinds of lime.' 

 This action he attributes, after Sir Humphry Davy, to the longer period of 

 causticity commonly supposed to occur with magnesia. In the form of car- 

 bonate, he says, 'magnesia seems to exercise a highly favorable action; and 

 magnesian limestone may perhaps be regarded as the most valuable of any, 

 since a smaller quantity of it suffices for the ends proposed.' 



"The subject is quite fully discussed by Storer (Agriculture, 1897, Vol. 

 2, page 135), who notes that it was early observed by English chemists that 

 certain limestones which had sometimes been found in practice to injure crops, 

 contained magnesia, and that Tennant, on applying calcined magnesia to 

 various soils with different crops, found that his plants either died, were un- 

 healthy, or vegetated very imperfectly ; also, that Knop found, in growing plants 

 by water culture (i.e., in very dilute solutions of plant foods), that magnesium 

 salts are distinctly harmful unless accompanied by abundance of lime, potash, 

 or ammonia salts; by themselves, the magnesium salts caused peculiar mal- 

 formations of the plant roots, followed shortly by the death of the plants. 



"Storer notes, on the other hand, that Sir Humphry Davy found that the 

 very magnesian limestones to which objection was made, gave very beneficial 

 results on certain soils, and that magnesia, though injurious when present in 

 caustic condition in considerable quantity in ordinary soils, may be beneficial 

 when mixed with peat or where present as carbonate. " 



In a recent investigation at Rothamsted, Ashby reports a larger 

 fixation of nitrogen by Azotobacter when magnesium carbonate 

 was present than when calcium carbonate was used. For each 



